The invention relates to a data coding system and especially, a data coding system for use in a passive transponder system.
A typical radio frequency (RF) transponder system consists of an interrogator or reader and one or several radio frequency transponders or tags. Such systems are used for remotely locating, identifying and tracking objects. Depending on the type of requirement, these transponder systems are used to read data stored in the tag or write data to the tag and in some cases to both read data from and write data to the tag.
Transponder systems are broadly classified into two categories: (i) active transponder systems; and (ii) passive transponder systems. Active transponder systems include a battery in the tag and the battery provides DC power for the tag either wholly or partly. Passive transponder systems do not have a battery in the tag. In passive transponder systems, DC power requirement for the tag is obtained from the electromagnetic waves emitted by the interrogator. The interrogator sends, depending on the type of system, either continuous or pulsed RF electromagnetic waves. These electromagnetic waves are received by an antenna in the tag and rectified to generate the DC power for the tag. The efficiency of this power transformation is low and is restricted by the performance of the rectifying device as well as other components in the tag. It is also restricted by a decrease in the received signal level as the tag moves away from the interrogator. Therefore, the operating distance or the range of the passive transponder system is dependent on the distance at which the RF signal received by the tag is high enough to generate sufficient DC power to operate the tag.
In operation, when the RF signal strength is high enough to operate the tag, a logic circuit or microcontroller within the tag receives the enquiry from the interrogator and generates a serial data signal representing the data that the tag is to send in response to the interrogator. This serial data signal is transmitted back to the interrogator via the tag antenna using the same RF signal emitted by the interrogator by backscatter or by coupling. The serial data signal is used by the tag to create a change in the loading of the antenna or resonance of the antenna or the impedance seen by the antenna in the tag according to the state of the data signal, in order to produce the backscatter. The interrogator antenna receives the backscattered response signal and the response signal is demodulated in the interrogator to recover the data sent by the tag.
During the period when the tag sends the response data signal, two simultaneous activities take place in the tag:
1. The response data signal is sent by backscattering; and
2. The DC power requirement of the tag is generated from the RF signal emitted by the interrogator.
As these two activities are taking place simultaneously, the DC power transformation from the RF signal gets affected. The tag will be able to derive optimum DC power only when the antenna is optimally loaded or when it is resonant or when its impedance is matched since the tag receives optimum electromagnetic waves only under this condition. Thus when the loading of the antenna or its resonance or the impedance seen by the antenna is changed in accordance with the data signal in order to produce backscatter, the tag will receive optimum electromagnetic waves only for the state of the data signal which produces optimum loading of the antenna or resonance of the antenna or when the antenna impedance is matched. During the other state of the data signal, the tag will not receive optimum electromagnetic waves. Thus the RF power received by the tag depends on the duration of the state of the response signal which results in optimum electromagnetic waves received by the tag and this in turn determines the RF to DC power transformation in the tag. During the period of the data signal when the tag does not receive optimum electromagnetic waves, the passive tag relies on charge stored in a reserve capacitor of the rectifier circuit to supply its necessary DC power requirement.
In prior art systems coding schemes like Manchester code and Miller code are used to represent the response data signal. In Manchester coded binary data, the bit whether it is a xe2x80x981xe2x80x99 or a xe2x80x980xe2x80x99 has a duty cycle of 50%. This means that both states have equal duration. Therefore, the response data signal will have a high state and a low state for almost equal duration. The Manchester coded waveform of a digital 1 and 0 are shown in FIGS. 4a and 4b, respectively. From FIGS. 4a and 4b it can be seen that for a duration of one bit, 50% of the duration consists of State1 (or the high state) and the remaining 50% consists of State2 (or the low state). This is a limitation and because of this the RF to DC power transformation is not efficient, as the power transformation is higher only for one of the states of the symbols, and so for only approximately 50% of the signal. When the data sequence has bits digital 1 and 0 coming together as 10 or 01, as shown in FIG. 5, the state of the waveform remains high or low as the case may be for a full one bit duration. In this case the RF to DC power transformation is not efficient for the full duration of the bit period. Other data coding schemes are similar as far as the duration of the two states is concerned.
U.S. Pat. No. 5,305,008 describes the use of an optimally configured rf voltage magnifying resonator circuit placed between the antenna and the DC rectifier circuit to enhance the effective interrogation range. The same patent also describes another method by which the interrogation range is further enhanced. This is by appropriate shaping of the envelope of the interrogating signal generated by the interrogator.
U.S. Pat. No. 5,930,303 describes an encoding method called aperture coding. In this method a data bit having a bit center is divided into plurality of time segments or apertures. The encoding is done by selecting a first subset from the plurality of apertures ending prior to the bit center and the second subset from the remaining plurality of apertures ending after the said bit center. This form of encoding has a bit center and apertures ending prior to the bit center is decoded as a digital 1 and apertures ending after the bit centre is decoded as a digital 0. This form of encoding is used to improve the spectral efficiency. However, the data sequence using this method of encoding will also have a fairly long duration of each of the states compared to the total duration of the data sequence.
In accordance with a first aspect of the present invention, there is provided a data coding system for coding data represented by a number of symbols, the system comprising representing each symbol with a unique digital waveform, all the unique digital waveforms having a duty cycle greater than 50% or a duty cycle less than 50%.
An advantage of the invention is that by providing a coding system in which each symbol is represented by a unique digital waveform and all the unique digital waveforms have a duty cycle greater than 50% or less than 50% it is possible to maximise the power transformation efficiency of a tag in a passive transponder system.
The term xe2x80x9cduty cyclexe2x80x9d as used herein is the ratio of the duration of a digital waveform xe2x80x9cONxe2x80x9d (or high state) time to the total period of the waveform.
Preferably, the unique digital waveforms differ from each other by changes in state at different points during the period of the waveform.
Typically, each unique digital waveform may have a unique duty cycle.
Preferably, the data is represented by binary symbols. However, that other symbols could be used to represent the data.
Typically, where the duty cycles are greater than 50%, one waveform has a duty cycle of 100%. Typically, where the duty cycles are less than 50%, one waveform has a duty cycle of 0%.
In accordance with a second aspect of the invention, there is provided a passive transponder system comprising a central unit and a radio frequency transponder, the radio frequency transponder, in response to a radio frequency interrogation signal from the central unit, coding data to be transmitted to the central unit using a coding system in accordance with the first aspect of the invention.
Preferably, the duty cycles of the digital waveforms are greater than 50% if the high state of the waveform provides better RF to DC power conversion than the low state, and typically, the duty cycles are maximised. Preferably, the duty cycles of the waveforms are less than 50% if the low state of the waveform provides better RF to DC power conversion, and typically the duty cycles are minimised.
The difference between the duty cycles is kept as small as possible to ensure that the bit duration of both digital 1 and digital 0 will be dominated by one state whether it is a high state or a low state. But at the same time the difference should be sufficient enough to ensure proper decoding of the data at the interrogator. A high duty cycle will ensure that the state of the bit will remain high for most of the bit duration whether it is representing a digital 1 or a digital 0. On the other hand a low duty cycle will ensure that the state of the bit will remain low for most of the bit duration whether it is representing a digital 1 or a digital 0. For encoding, the decision of whether to go for the high duty cycle or the low duty cycle will depend on which state of the bit produces optimum loading of the tag antenna or resonance of the tag antenna or optimum matching to the tag antenna impedance. This ensures that the tag receives optimum RF waves for most of the bit duration and the duration of the bit period during which the RF waves received by the tag is not the optimum remains very small. This method of encoding improves the efficiency of the RF to DC power transformation when the passive transponder is sending its response signal.